1. Field of the Invention
The present invention relates generally to a motor-driven compressor and, in particular but not exclusively, to the prevention of poor insulation between a power supply terminal and a metallic housing of the motor-driven compressor.
2. Description of the Related Art
FIG. 1 depicts a conventional motor-driven compressor 1. As shown therein, the motor-driven compressor 1 includes an electric motor 2 and a compression mechanism 4, both accommodated in a metallic shell or housing 6. When the compression mechanism 4 is driven by the electric motor 2, gas refrigerant drawn into the shell 6 through a suction pipe 6a is compressed and then discharged through a discharge pipe 6b. The electric motor 2 is supplied with electric power from outside via a power supply terminal 10 secured to an end face of the shell 6.
FIG. 2 depicts the structure of the power supply terminal 10. As shown therein, the power supply terminal 10 includes a metallic terminal base 14 secured to the shell 6 and a plurality of pins 11 secured to the terminal base 14 via a glass insulator 16 and a ceramic insulator 18 for electrical insulation. A tab 12, connected to the electric motor 2 via a lead wire 13, is secured to each of the pins 11.
A relatively high voltage is applied to the power supply terminal 10. By way of example, in applications where the electric motor 2 is supplied with electricity from a 100 V, 60 Hz power source, a voltage of about 60 V is applied to the power supply terminal 10. Further, an increased voltage is applied with an increase in frequency for driving the electric motor 2. On the other hand, the terminal base 14 is grounded via the shell 6. Accordingly, a large potential difference is created between the pins 11 and the terminal base 14 and, hence, high electrical resistance is required to maintain assured electrical insulation between the pins 11 and the terminal base 14. Particularly, in motor-driven compressors for use in electric cars or hybrid cars, high insulation resistance greater than 10 Mxcexa9 is generally required for enhanced safety.
However, the above-described conventional motor-driven compressor 1 entails a problem that the insulation resistance between the pins 11 and the terminal base 14 may become insufficient depending on the state of internal refrigerant. Although only gas refrigerant circulates within the motor-driven compressor 1 during normal operation, when the motor-driven compressor 1 is stopped, the gas refrigerant remaining therein is cooled, and there is a good chance that liquefied refrigerant is still left within the compressor. Because the liquefied refrigerant has a specific resistance smaller than the gas refrigerant, when the power supply terminal 10 is wet with or in some cases submerged under the liquid refrigerant, the insulation resistance between the pins 11 and the terminal base 14 is reduced to, for example, about 1 Mxcexa9 or less. When the motor-driven compressor 1 is operated under such conditions, it is likely that electric current supplied to the power supply terminal 10 leaks considerably to the metallic shell 6 through the terminal base 14. Particularly, in the case of the horizontal compressor shown in FIG. 1, in which the power supply terminal 10 is attached to an end face thereof, the power supply terminal 10 is apt to become wet with liquefied refrigerant stored therein and, hence, there is a good chance that poor insulation occurs between the pins 11 and the terminal base 14.
The present invention has been developed to overcome the above-described disadvantages.
It is accordingly an objective of the present invention to provide a power supply terminal that is suited for use with a motor-driven compressor and can prevent poor insulation between it and a metallic housing of the motor-driven compressor.
Another objective of the present invention is to provide a method of insulating the power supply terminal from the metallic housing of the motor-driven compressor.
In accomplishing the above and other objectives, the power supply terminal includes a base secured to the metallic housing, a conductive element secured to the base, an insulator for insulating the conductive element from the base, and an insulating resinous cover for covering a portion of the conductive element and a portion of the insulator that are located inside the metallic housing.
This construction elongates the shortest distance between the conductive element and the base or reduces the cross section of a current leakage path, making it possible to prevent poor insulation between the power supply terminal and the metallic housing.
It is preferred that the insulating resinous cover is in the form of a tube having an inner diameter for allowing the conductive element and the insulator to be inserted thereinto. The use of the tube-shaped insulating resinous cover facilitates the covering work for the power supply terminal and maintenance work such as replacement work of a lead wire.
Advantageously, the insulating resinous cover is made of a heat-shrinkable material such, for example, as a fluorine-based resin. The heat-shrinkable cover can be readily held in close contact with the insulator when heated, thus enhancing the insulation resistance between the conductive element and the base. The cover made of a fluorine-based resin has good durability with respect to both refrigerant and oil, enhancing the reliability of the compressor.
The motor-driven compressor may be a horizontal one having an end face to which the power supply terminal is secured. In the case of the horizontal compressor, although the power supply terminal is occasionally submerged in liquid refrigerant, the insulating resinous cover acts to prevent poor insulation.
In another aspect of the present invention, a method of insulating a power supply terminal from a metallic housing of a motor-driven compressor includes the steps of: (a) moving a heat-shrinkable resinous tube towards the power supply terminal so that a portion of the conductive element and a portion of the insulator that are located inside the metallic housing are covered with the heat-shrinkable resinous tube, (b) inserting a conductive element connector into an opening of the heat-shrinkable resinous tube and connecting the conductive element connector to the conductive element, and (c) heating the heat-shrinkable resinous tube to shrink the heat-shrinkable resinous tube.
According to this method, a portion of the power supply terminal that is located inside the metallic housing can be easily covered with the resinous tube without performing new processing with respect to the parts that have been hitherto used.
Conveniently, before the step (b), a notch is formed in the heat-shrinkable resinous tube so that a lead wire, which is connected to the conductive element connector so as to extend therefrom in a direction perpendicular thereto, is inserted into the notch during the step (b). The provision of such a notch facilitates the connection of the L-shaped conductive element connector to the conductive element.